Frequency diversity time multiplex means for increasing the capacity of a cooperative collision avoidance system

ABSTRACT

A frequency diversity time multiplex means for increasing the capacity of a cooperative collision avoidance system wherein the carrier frequency of transmissions in adjacent time slots is varied stepwise on a predetermined schedule.

waited States Patent 1 [111 3,757,339

Shear et al. *Sept. 4, 1973 FREQUENCY DIVERSITY THME [58] Field ofSearch 343/112 CA, 112 TC MULTIPLEX MEANS FOR INCREASING 340/23 THECAPACITY OF A COOPERATWE COLLISION AVOIDANCE SYSTEM [56] ReferencesCited [75] Inventors: Wayne G. Shear, Pompano Beach, UNITED TATESPATENTS Fl3..; Merlin E. Olmstead, Baltimore, 3,55l,884 l2/l970 Shear etal 340/23 Md. 3,573,8l8 4/l97l Lennon, Jr. et a]. 343/112 TC X A h B thf[73] Sslgnee Q z Comm-anon Sou leld Primary Examiner-Benjamin A.Borchelt I Assistant ExaminerRichard E. Berger [*1 Notice: The portionof the term of this patent subsequent to Dec. 29, 1987 Lamb and WilliamChristofom has been disclaimed. [22] Filed: Oct. 21, 1970 AttmeyPlante,Hartz, Smith & Thompson, Bruce L.

[57] ABSTRACT PP N05 82,645 A frequency diversity time multiplex meansfor increas- Related Application Data ing the capacity of a cooperativecollision avoidance Continuation ofser No 694 239 Dec 28 196.7 Patsystemwherein the carrier frequency of transmissions No. 3 551 884 in adjacenttime slots is varied stepwise on a predetermined schedule.

[52] US. Cl. 343/112 CA, 340/23, 343/112 TC [51] Int. Cl 608g 5/04Claims. 2 Drawing Figures RF GENERATOR 3 MODULATOR ALTITUDE ALTITUDERATE ALTIMETER RANGE RAN RATE ALTITUDE ALTITUDE RATE MANEUVER COMMANDCOLLISION THREAT COMPUTER DECODER FREQUENCY DIVERSITY TIME MULTIPLEXMEANS FOR INCREASING TIIE CAPACITY OF A COOPERATIVE COLLISION AVOIDANCESYSTEM This is a continuation of copending application Ser. No. 694,239,flled Dec. 28, 1967, now U.S. Pat. No. 3,551,884.

The problem of preventing mid-air collision of aircraft has longconfronted the aviation industry generally. Recently this problem hasbecome acute with the introduction of relatively large and expensiveaircraft carrying large numbers of passengers in each aircraft. Thepublic has come to expect that the commercial aviation industry providea comprehensive flight schedule carrier out with a high degree of safetyand the industry has striven to produce this. It is, however, nowrecognized that the capacity of the conventional air trafflc controlsystem (ATC) suffers from rather severe limitations because of theincreasing volume of air travel. Under conventional ATC concepts eachaircraft is assigned an exclusive volume of air space. As the density oftrafflc increases and air speeds increase, the volume of space whichmust be assigned to each aircraft becomes larger. With more aircraft inthe air the available air space may become exhausted leaving asalternatives either a reduction in the assigned volume to each aircraftor a curtailment of flight services below that for which there isdemand.

If the position of an airborne aircraft could be determined with greateraccuracy with respect to other aircraft in its vicinity and the aircraftcontrolled with respect to this information, the effective available airspace could be increased several fold. While it is conceivable that thiscan be accomplished with ground based equipment, the present indicationis that the required accuracy and aircraft control cannot be achievedthereby. The practical solution to the problem, therefore, appears to bethe equipping of each aircraft with suitable equipment to warn the pilotof a potentially dangerous situation with respect to nearby aircraft.The goal for many years has been to develop a self-contained collisionavoidance system, wherein the equipped aircraft would be capable withoutany external aid to determine when a collision with another aircraft wasa possibility and take the necessary steps to avoid the collision. Theattractiveness of a selfcontained system is primarily due to the conceptthat the equipped aircraft would be able to protect itself withouthaving to rely on all aircraft in the sky being suitably equipped andhence, where safety was desired it could be purchased without therequirement that other aircraft be so equipped. A proposedselfcontained, independently operating system comprised a computer whichpredicted an impending collision from bearing and range informationobtained from an accurate short range radar. Attractive as theselfcontained system is, the concept has proved to be impractical, inthat an active radar with sufflcient angular resolution to predict animpending collision from bearing constancy information is beyond thepresent state of the art because of serious limitations imposed byground clutter, antenna size requirements, scanning losses, target blipscintillation, and available power output.

As an alternative, a cooperative collision avoidance system was proposedwherein each aircraft in the anticollision net was equipped withsuitable equipment including an altimeter, an encoder, a computer, atransmitter and a receiver. An intruder aircraft in a givenanti-collision locality transmitted its altitude which was derived fromthe altimeter, at randomly selected time intervals to diminish theprobability of interfering signals. All other aircraft in theanti-collision net received this information, both via a straightlinetransmission path and also via ground bounce. The time difference inreception of the straight line signal as opposed to the ground pathsignal when combined with the altitude of the receiving aircraft and thealtitude of the transmitting aircraft allowed the receiving aircraft toderive the range of the transmitting aircraft. After a number of rangeshad been computed by the receiving aircraft it could additionallycompute a range rate. The ratio of range to closing range-rate, which isdeflned as the TAU function, is one criterion of collision threat. Theeffectiveness of TAU as a collision threat predictor is dependent uponhow close TAU approximates the real time to closest approach (T) of thetransmitting aircraft to the receiving aircraft. Analysis has shown, andit is well known in the art, that for large values of TAU at longranges, TAU is a good approximation of T, being equal to T where thecourses and speeds of the aircraft are such as to cause an actualcollision. Also, when the relative velocity between the aircraft islarge, TAU is a good approximation to T down to a predetermined alarmthreshold value of TAU. However, when the aircraft are on slowlyconverging courses so that the closing range rate is small, TAU becomesquite large and cannot be considered in these circumstances as a validthreat criterion. A supplemental collision threat criterion based onminimum range must, therefore, be used. It will be remembered that bothclosing range rate and range are available so that a complete collisionthreat evaluation could be made. Two problems have shown this system tobe impractical. First, since successive computation of ranges isrequired before a range rate can be computed, some comparatively longperiod of time must elapse from the time of flrst receipt of a collisionavoidance message until the computer can make an evaluation of thecollision threat. In practical equipment, approximately 20 seconds ofdata processing was required to compute range rate. The second problemis caused by the short time interval between the receipt of a directsignal and the bounce signal at low altitudes which can introduce alarge uncertainty into the range calculation.

One of the more recently proposed collision avoidance systems whichappears to have the best chance of providing a practical collisionavoidance network also uses the TAU and range criteria outlined above.This system utilizes a so-called master time technique wherein eachcooperating aircraft is equipped with an accurate clock which issynchronized with all other airborne clocks in the anti-collision netand additionally may be synchronized with a master ground clock. A 5second long epoch is divided into equally spaced time slots, eachaircraft in the collision net being assigned a given time slot. Assumingall clocks in the net to be synchronized at the beginning of an epoch,all airborne systems simultaneously transmit a start signal at thebeginning of the epoch. Thereafter, at its assigned time slot, anaircraft will transmit a collision avoidance message containinginformation as to its altitude rate and altitude. The frequency on whichthis message is transmitted is controlled in a predetermined manner bythe clock so that the transmitter frequency is known to all otheraircraft; therefore, a doppler shift in the received frequency at thereceiving aircraft is a measure of the range rate of the transmittingaircraft with respect to the receiving aircraft. Additionally, since thetime at which the transmitting aircraft commenced its transmission isknown, the time of the message reception is a measure of the rangebetween the transmitting aircraft and the receiving aircraft. It hasbeen determined that typically an 800 microsecond period is required totransmit the collision avoidance system message. The present cooperativesystem is designed to warn of a collision threat at a range of 60 miles.However, when considering certain conditions of the antenna radiationpattern, transmittal power, receiver sensitivity, etc. the system willbe likely to react to signals at 600 miles which is the line of sightlimitation for two aircraft at 60,000 feet altitude. The probability ofline of sight interference, therefore, becomes a very real problem whichmust be considered in the definition of a practical collision avoidancesystem, including cooperative systems based on the aforementionedstandard time'- frequency techniques. Normally a time slot would have tobe sufficiently long to allow transmission of the collision avoidancesystem message, plus the transit time of the message to a possiblereceiver. The 600 mile line of sight of transit time is approximately3.7 milliseconds. This time added to a data period of approximately 800microseconds implies a slot period of 4.5 milliseconds if adjacent slotinterference between aircraft having the aforementioned 600 mile line ofsight limitation is to be precluded. Assuming an epoch period of fiveseconds, it is obvious that only 1,100 such 4.5 millisecond slots can beaccommodated on a single r.f. channel. Not only is this number of timeslots clearly insufficient to accommodate future projected aircraftentities, it is now in danger of being overloaded in certain highdensity environments, even assuming stringent regional time slotmanagement.

SUMMARY OF THE INVENTION Accordingly, a frequency diversity timemultiplex technique and means has been devised for providing anapproximately four-fold increase in capacity in a given epoch time,wherein the slot period is determined by the data transmission periodplus the maximum design range time rather than the data transmissionperiod plus the maximum line of sight transmission time. Thus, assumingapplication of a message structure 800 microseconds in length and a 60mile range through which it is desired to transmit this message, theslot need be only 800 microseconds for the message plus 382 microsecondsfor the 60 mile maximum design range, meaning a slot period of only1.172 milliseconds is required. Adjacent slot line of sight interferenceis eliminated by stepping the frequency at the end of a useful slotperiod cyclically through four distinct frequencies, one for each offour consecutive time slots and then repeating the program for the nextfour consecutive time slots, etc. Thus the transmitter assigned slot onewould transmit on frequency one and all receivers would receive onfrequency one; the transmitter assigned slot two would transmit onfrequency two and all receivers would receive on frequency two;transmitter three on frequency three and transmitter four on frequencyfour, following which the sequence would be repeated,

that is, transmitter five would transmit on frequency one, transmittersix on frequency two, etc. This technique of eliminating adjacent slotinterference by stepping the frequency at the end of the useful slotperiod allows a total of 4,250 slots in a five second epoch. It will benoted that the time interval required to step through four consecutivetime slots is 4.688 milliseconds or slightly in excess of the timerequired for a 600 mile line of sight transmission. Since, as has beenmentioned, 600 miles is the line of sight limitation for two aircraft at60,000 feet altitude the possibility of signal interference iseliminated for aircraft operating below this altitude and made highlyimprobable for aircraft operating over this altitude.

This system, while providing a four-fold increase in capacity in a givenepoch time, requires very little increase in complexity. All that isrequired is that the receiver local oscillator be stepped in frequencyin synchronization with the slot counts and similarly that thetransmitters be able to transmit the same four frequencies, since theaircraft slot assignment will determine the carrier frequency of thetransmission.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram of the structure ofa collision avoidance system message based on the teachings of thisinvention.

FIG. 2 is a block diagram of an airborne station in accordance with thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 thecollision avoidance system message which a cooperating aircrafttransmits in its time slot is seen to consist of a 200 microsecond c.w.pulse 10 which is required for doppler range rate determination. Theleading edge of the 200 microsecond doppler pulse, when compared withthe master clock in the receiving aircraft, serves to determine therange to the transmitting aircraft from the receiving aircraft. Amulti-path guard time of 170 microseconds 12 is provided after thedoppler pulse to eliminate ground interference. The guard time includesmicroseconds for a 60,000 foot altitude round trip, plus 50 microsecondsringing time. An altitude rate pulse 14, typically 2 to 4 microsecondslong, is next transmitted, the leading edge of which, with respect tothe leading edge of the doppler pulse, communicates the altitude rate ofthe transmitting aircraft. An altitude rate pulse leading edge occurringless than 400 microseconds after the doppler pulse leading edgeindicates decreasing altitude, while a similar leading edge occurringmore than 400 microseconds after the doppler pulse indicates increasingaltitude and a leading edge occurring 400 microseconds after the dopplerpulse leading edge indicates zero altitude rate. Additionally, thefollowing altitude rate pulse position code is recommended:

Altitude Weighing Pulse Leading Edge Deviation from 400 Microsecv RateFactor 0 to 2K Ft./Min. 250 FtJMinJpsec. 0 to Spsec.

2 to 5K Ft./Min. 500 Ft./Min./p.sec. 9 to l4psec.

5 to 20K FtJMin. I000 FtJMinJpsec. 15 to 29usec.

As an example of altitude rate determination, assume an altitude ratepulse is received 385 microseconds after a doppler pulse leading edge.This is a l 5 microsecond deviation from 400 microseconds slot time. The

transmitting aircraft's altitude can thereby be calculated to bedecreasing at a rate of 250 FL/Min/usec. for the first 8 microseconds ofdeviation or 2,000 Ft./Min. plus 500 Ft./Min./,usec. for the next 9 to14 microseconds of deviation or 3,000 Ft./Min. additional plus 1,000Ft./Min./p.sec. for the last microsecond deviation for a total of 6,000FtJMin. A maximum altitude rate of 20,000 Ft./Min. is to be expected,thus the altitude rate pulse should occur within the time interval of370 to 430 microseconds after the doppler pulse leading edge.

Another multi-path guard time of 170 microseconds follows the altituderate pulse and is followed in turn by an altitude pulse 18 alsotypically 2 to 4 microseconds long. The position of the altitude pulseleading edge with respect to the leading edge of the doppler pulseindicates the altitude of the transmitting aircraft with zero altitudebeing indicated by a pulse commencing 600 microseconds after zero slottime. The following altitude code is recommended:

Altitude Weighing Pulse Leading Edge Deviation from 600 psec.

. Ractor O to 20K Ft. 250 Ft./p.sec. 0 to 80 psec.

20K to 50K Ft. 500 Ft./p.sec. 81 to 140 psec.

50K to 100K Ft. 1000 Ft./p.sec. Over I40 usec.

Computing the transmitting aircrafts altitude in the same manner thataltitude rate is computed, an altitude pulse having a leading edge at750 microseconds slot time indicates an altitude of 20K Ft. plus 30K Ft.plus K Ft. for a total of 60,000 feet. Aircraft altitudes under 100,000feet are to be expected to occur between 600 and 790 microseconds afterthe doppler pulse leading edge. The time slot continues for another 382microseconds which corresponds to the transit time for slightly inexcess of a 60 mile range. The slot period is thus seen to be 1,172microseconds, at the end of which the frequency is stepped to eliminateadjacent slot line of sight interference.

Although it has been shown that four frequencies is optimum for themessage length and line of sight protection desired, a different messagelength or line of sight protection might dictate a different number offrequencies.

It should be remembered that an aircraft is assigned only a single timeslot in a cyclical epoch which is approximately 5 seconds long,therefore, each aircraft transmits its collision avoidance message onceevery 5 seconds. However, since all factors determinitive of a collisionthreat are conveyed by the single collision avoidance message, themaximum time required to make a collision threat evaluation after anintruder aircraft comes within message range of a receiving (protected)aircraft is 5 seconds where the intruder comes within message rangeimmediately after the termination of his assigned time slot.

Referring to FIG. 2, r.f. generator which is suitably a frequencysynthesizer and is a part of the transmitter section ofa single localunit in a collision avoidance system generates four r.f. carrierfrequencies, f,, f f and f which are suitable for use in a collisionavoidance system network. The r.f. frequencies are applied respectivelyto gates 22, 24, 26 and 28. A clock 30 containing suitable countingcircuits plus a controlling cesium atomic clock which has beensynchronized with all other clocks in the collision avoidance system,supplies a reference frequency to r.f. generator 20 to precisely controlits output frequencies and additionally generates gate enabling pulsessequentially on lines 30A, 30B, 30C, and 30D. Each gate enabling pulsehas a period equal to the period of a single time slot. The outputs ofgates 22, 24, 26 and 28 are applied to OR gate 31 with output from thatgate applied to AND gate 32, which is enabled by a clock generated gateenabling pulse appearing on line 30E, the duration of this enablingpulse being the period of one time slot, the pulse commencing with thebeginning of the time slot assigned to this particular unit. The carrierfrequency passing through gate 32 is amplified in amplifier 33 and thenapplied to modulator 38. Simultaneously with the opening of gate 32,information is applied by the clock through line 30F to pulse positionmodulator 38 for the purpose of positioning the doppler, altitude andaltitude rate pulses with respect to slot zero time. The leading edge ofthe altitude pulse is set by altitude information received fromaltimeter 42, while the leading edge of the altitude rate pulse is setby altitude rate information received from the altimeter which issuitably a doppler altimeter generating both altitude and altitude ratesignals. The output of gate 32, which is the carrier frequency, is thusmodulated and then transmitted over antenna 40.

The receiver section of the unit comprises local oscillators 54, 56, 58and 60, whose outputs are applied respectively to AND gates 62, 64, 66and 68. The receiver AND gates are energized by the same enabling pulsesas energize the transmitter AND gates, gates 22 and 62 being energizedsimultaneously and in like manner gates 24 and 64, gates 26 and 66, andgates 28 and 68 are also energized simultaneously. Collision avoidancemessages are received on antenna 50 and applied to mixer 52. Assumingthat the message received is on frequency f since all clocks in thesystem are synchronized, clock line 30A will be energized, opening gates22 and 62. Local oscillator frequency 1 passes through gate 62 and ismixed with received radio frequency f in mixer 52, the resultantintermediate frequency being amplified in i.f. amplifier 53 and thenapplied to decoder 74. Additionally, the leading edge of the receiveddoppler pulse is compared with the start of the time slot as determinedby clock 30 and supplied to the decoder via line 74A to generate therange between the transmitting and receiving aircraft, while the leadingedges of the altitude and altitude rate pulses are compared with theleading edge of the received doppler pulse t to generate thetransmitting aircrafts altitude and altitude rate. Decoder 74 typicallyincludes a crystal phase shift discriminator for determining the phaseshift of the received doppler pulse, and hence the range rate of thetransmitting aircraft. Additionally, the decoder suitably includes threecounters, the first of which is triggered into counting by own unitsstart of time slot signal supplied from the clock over line 74A and isterminated by the leading edge of a received doppler pulse, whichreceived leading edge triggers the second and third counters intocounting. The counts of the second and third counters are terminated bythe receipt of the altitude rate and altitude pulses respectively.Weighting networks on the counter outputs generate voltage signalsproportional to range, altitude rate and altitude, while the phase shiftdiscriminator output is a voltage signal proportional to range from thereceiving to transmitting aircraft.

Similarly, at the start of the next time slot line 308 is energized sothat gates 24 and 64 are open. If a collision avoidance system unit isassigned to that time slot and is transmitting its message, the messagewill be received on antenna 50 and mixed with local oscillator frequency2. The mixed products, as before, are applied to decoder 74 so as togenerate the collision avoidance information with respect to thereceiving aircraft and the aircraft then transmitting its collisionavoidance message. The local oscillator frequencies are, of course,off-set with respect to one another by an amount sufiicient to maintainintermediate frequency output of mixer 52 constant. The localoscillators are stabilized by a reference signal supplied by the clock.

The outputs of decoder 74, namely the intruders range, range rate,altitude and altitude rate, are supplied to collision threat computer 76which evaluates the information received and issues a maneuver commandto the pilot should it find that a collision threat exists. As has beendiscussed, the collision threat can be evaluated by examining TAU andrange. In order to prevent unnecessary maneuver commands and to decidewhat type of maneuver command should be issued if the collision threatdoes exist, the collision threat computer 76, after determining that theTAU or range criteria indicate that a collision threat exists, willcompare the intruders altitude and altitude rate with own aircraftaltitude and altitude rate to further determine whether at the predictedtime of closest approach the intruder will be within a predeterminedvertical distance to own aircraft. If this additional criterion is alsomet so that the intruder will be within own aircrafts protected verticaldistance at the time of closest approach, the maneuver command will beissued. Three basic maneuver commands have been proposed:

1. Climb/Descend 2. Hold Altitude 3. Level Off 4. Roll Out (Return tolinear flight) The actual operation and constructional details of thecollision threat computer are not a part of the present invention, thiscomputer being shown only to indicate the manner in which the derivedrange, range rate, altitude and altitude rate signals can be combinedwith own aircrafts altitude and altitude rate to determine theprobability of collision and the evasive maneuver required to decreasethat probability.

Although we have shown what we consider to be the preferred embodimentof our invention, certain alterations and modifications will becomeapparent to one skilled in the art. We do not wish to limit ourinvention to the specific form shown and accordingly hereby claim as ourinvention the subject matter including modifications and alterationsthereof encompassed by the true scope and spirit of the appended claims.

What is claimed is:

1. In a collision avoidance system wherein collision avoidance messagesare transmitted by various units within the system during timedetermined time slots and wherein each time slot is assigned one out ofaplurality of different frequencies upon which a unit transmitting duringthat time slot will transmit its collision avoidance message, receivermeans comprising:

clock means for keeping account of said time slots and for generatingclock signals; and,

a receiver qualified by said clock signals to receive said transmittedcollision avoidance messages.

2. Receiver means as recited in claim 1 wherein said receiver comprises:

local oscillator generator means responsive to said clock signals forgenerating a plurality of local oscillator frequencies, one at a time,each said local oscillator frequency being correlated to a particulartime slot and being generated during that time slot; and,

means responsive to said transmitted collision avoidance messages andthe local oscillator frequency being generated for receiving saidcollision avoidance messages.

3. Receiver means as recited in claim 1 and including an intermediatefrequency means responsive to a receiver intermediate frequency forreceiving said collision avoidance messages and wherein said receivercomprises:

means responsive to said clock signals for generating a plurality oflocal frequencies, one at a time, each said local frequency beingassociated with a particular time slot and being generated during thattime slot, one of the mixed frequencies of any local frequency with thefrequency assigned to its associated time slot being said receiverintermediate frequency; and,

means responsive to said transmitted collision avoidance messages andsaid local frequencies for generating said receiver intermediatefrequency.

4. A collision avoidance receiver comprising:

a source of clock signals;

local oscillator means responsive to said clock signals for generating aplurality of local oscillator frequencies on a predetermined timeschedule;

means for receiving transmitted collision avoidance messages; and,

means responsive to said generated local oscillator frequencies forextracting collision avoidance information from said received collisionavoidance messages.

5. A collision avoidance receiver as recited in claim wherein said localoscillator means comprises:

first means for generating a first local frequency;

second means for generating at least a second local frequency; and,

means responsive to said clock signals for energizing only said secondmeans during a second predetermined time period, said first and secondlocal frequencies being two of said plurality of local oscillatorfrequencies.

6. In a transmitter of a collision avoidance system for transmitting acollision avoidance message from one unit of said system to other unitsof said system wherein said one unit transmits its collision avoidancemessage via said transmitter during a unique time slot assigned to saidone unit in a system epoch on a frequency assigned to said unique timeslot, said assigned frequency being one of a plurality of frequenciesassigned to said system said one unit including clock means for countingand timing said time slots and for generating signals in accordancetherewith, an improvement comprising means responsive to said clocksignals for generating said first frequency during said unique timeslot,

7. The collision avoidance system and means recited in claim 6 withadditionally receiver means comprising:

local oscillator means responsive to said clock signals for generating aplurality of local oscillator frequencies, each said local oscillatorfrequency corextracting collision avoidance information en-' codedthereon.

8. A collision avoidance transmitter comprising:

a source of clock signals;

means responsive to said clock signals for generating a frequency signalduring a predetermined time period;

means for generating signals correlated to the location of saidtransmitter;

means for modulating said frequency signal with said location signals;and,

means for transmitting said modulated frequency signal.

9. A collision avoidance transmitter as recited in claim 8 wherein saidmeans for generating signals correlated to the location of saidtransmitter comprises means for generating signals correlated to atleast the altitude of said transmitter.

10. A collision avoidance transmitter as recited in claim 8 wherein saidpredetermined time period is a time slot in a collision avoidance systemepoch, said frequency signal being assigned, out of a plurality offrequencies, to said time slot.

11. In a collision avoidance system for transmitting and receivingcollision avoidance messages between individual units of said systemwherein each transmitting unit within said system transmits a collisionavoidance message during a unique time slot assigned to said unit on oneradio frequency of a plurality of radio frequencies assigned to saidsystem, said one radio frequency being assigned to said unique timeslot, said unique time slot being one of a plurality of time slots in asystem epoch, a transmitter associated with said unit comprising:

clock means for counting and timing said time slots and for generatingclock signals; and,

means responsive to said clock signals for generating and transmittingunits collision avoidance message during said units assigned time sloton said time slots assigned radio frequency.

12. In a collision avoidance system as recited in claim 11 additionallymeans for receiving collision avoidance messages transmitted from remoteunits during various time slots on various radio frequencies assigned tosaid time slots comprising:

means responsive to said clock signals for generating a plurality oflocal frequencies, one said local frequency being associated with eachof said plurality of radio frequencies, said one local frequency beinggenerated during the time slot of its associated radio frequency; and,

means responsive to said local frequencies for receiving said collisionavoidance messages.

13. A collision avoidance system receiver comprising:

means for generating clock signals;

means responsive to said clock signals for generating a plurality oflocal oscillator signals, one at a time, on a time ordered schedule;

means for receiving transmitted collision avoidance messages; and,

means responsive to said local oscillator signals for demodulating saidreceived collision avoidance messages.

14. In a collision avoidance system wherein time is divided into systemepochs and said epochs are further divided into time slots and wherein aplurality of radio frequencies are available for assignment one to atime slot whereby at least adjacent time slots are assigned differentones of said available radio frequencies, a transmitter assigned to aunique time slot for use in said system comprising:

means for generating the radio frequency assigned to said unique timeslot during said unique time slot;

a source of collision avoidance information;

means for modulating said generated radio frequency with said collisionavoidance information; and, means for transmitting said modulated radiofrequency.

15. A transmitter as recited in claim l4 wherein said generating meanscomprises:

clock means for keeping account of said epochs and time slots and forgenerating clock signals in accordance therewith; and,

means responsive to said clock signals for generating said radiofrequency assigned to said unique time slot during said unique timeslot.

1. In a collision avoidance system wherein collision avoidance messagesare transmitted by various units within the system during timedetermined time slots and wherein each time slot is assigned one out ofa plurality of different frequencies upon which a unit transmittingduring that time slot will transmit its collision avoidance message,receiver means comprising: clock means for keeping account of said timeslots and for generating clock signals; and, a receiver qualified bysaid clock signals to receive said transmitted collision avoidancemessages.
 2. Receiver means as recited in claim 1 wherein said receivercomprises: local oscillator generator means responsive to said clocksignals for generating a plurality of local oscillator frequencies, oneat a time, each said local oscillator frequency being correlated to aparticular time slot and being generated during that time slot; and,means responsive to said transmitted collision avoidance messages andthe local oscillator frequency being generated for receiving saidcollision avoidance messages.
 3. Receiver means as recited in claim 1and including an intermediate frequency means responsive to a receiverintermediate frequency for receiving said collision avoidance messagesand wherein said receiver comprises: means responsive to said clocksignals for generating a plurality of local frequencies, one at a time,each said local frequency being associated with a particular time slotand being generated during that time slot, one of the mixed frequenciesof any local frequency with the frequency assigned to its associatedtime slot being said receiver intermediate frequency; and, meansresponsive to said transmitted collision avoidance messages and saidlocal frequencies for generating said receiver intermediate frequency.4. A collision avoidance receiver comprising: a source of clock signals;local oscillator means responsive to said clock signals for generating aplurality of local oscillator frequencies on a predetermined timeschedule; means for receiving transmitted collision avoidance messages;and, means responsive to said generated local oscillator frequencies forextracting collision avoidance information from said received collisionavoidance messages.
 5. A collision avoidance receiver as recited inclaim 4 wherein said local oscillator means comprises: first means forgenerating a first local frequency; second means for generating at leasta second local frequency; and, means responsive to said clock signalsfor energizing only said second means duriNg a second predetermined timeperiod, said first and second local frequencies being two of saidplurality of local oscillator frequencies.
 6. In a transmitter of acollision avoidance system for transmitting a collision avoidancemessage from one unit of said system to other units of said systemwherein said one unit transmits its collision avoidance message via saidtransmitter during a unique time slot assigned to said one unit in asystem epoch on a frequency assigned to said unique time slot, saidassigned frequency being one of a plurality of frequencies assigned tosaid system said one unit including clock means for counting and timingsaid time slots and for generating signals in accordance therewith, animprovement comprising means responsive to said clock signals forgenerating said first frequency during said unique time slot.
 7. Thecollision avoidance system and means recited in claim 6 withadditionally receiver means comprising: local oscillator meansresponsive to said clock signals for generating a plurality of localoscillator frequencies, each said local oscillator frequency correlatedto one of said plurality of frequencies assigned to said system, onemixed frequency product of any local oscillator frequency with itscorrelated frequency being equal to the intermediate frequency of saidreceiver means; and, means responsive to said intermediate frequency forextracting collision avoidance information encoded thereon.
 8. Acollision avoidance transmitter comprising: a source of clock signals;means responsive to said clock signals for generating a frequency signalduring a predetermined time period; means for generating signalscorrelated to the location of said transmitter; means for modulatingsaid frequency signal with said location signals; and, means fortransmitting said modulated frequency signal.
 9. A collision avoidancetransmitter as recited in claim 8 wherein said means for generatingsignals correlated to the location of said transmitter comprises meansfor generating signals correlated to at least the altitude of saidtransmitter.
 10. A collision avoidance transmitter as recited in claim 8wherein said predetermined time period is a time slot in a collisionavoidance system epoch, said frequency signal being assigned, out of aplurality of frequencies, to said time slot.
 11. In a collisionavoidance system for transmitting and receiving collision avoidancemessages between individual units of said system wherein eachtransmitting unit within said system transmits a collision avoidancemessage during a unique time slot assigned to said unit on one radiofrequency of a plurality of radio frequencies assigned to said system,said one radio frequency being assigned to said unique time slot, saidunique time slot being one of a plurality of time slots in a systemepoch, a transmitter associated with said unit comprising: clock meansfor counting and timing said time slots and for generating clocksignals; and, means responsive to said clock signals for generating andtransmitting unit''s collision avoidance message during said unit''sassigned time slot on said time slot''s assigned radio frequency.
 12. Ina collision avoidance system as recited in claim 11 additionally meansfor receiving collision avoidance messages transmitted from remote unitsduring various time slots on various radio frequencies assigned to saidtime slots comprising: means responsive to said clock signals forgenerating a plurality of local frequencies, one said local frequencybeing associated with each of said plurality of radio frequencies, saidone local frequency being generated during the time slot of itsassociated radio frequency; and, means responsive to said localfrequencies for receiving said collision avoidance messages.
 13. Acollision avoidance system receiver comprising: means for generatingclock signals; means responsive to said clock signals for generating apluralIty of local oscillator signals, one at a time, on a time orderedschedule; means for receiving transmitted collision avoidance messages;and, means responsive to said local oscillator signals for demodulatingsaid received collision avoidance messages.
 14. In a collision avoidancesystem wherein time is divided into system epochs and said epochs arefurther divided into time slots and wherein a plurality of radiofrequencies are available for assignment one to a time slot whereby atleast adjacent time slots are assigned different ones of said availableradio frequencies, a transmitter assigned to a unique time slot for usein said system comprising: means for generating the radio frequencyassigned to said unique time slot during said unique time slot; a sourceof collision avoidance information; means for modulating said generatedradio frequency with said collision avoidance information; and, meansfor transmitting said modulated radio frequency.
 15. A transmitter asrecited in claim 14 wherein said generating means comprises: clock meansfor keeping account of said epochs and time slots and for generatingclock signals in accordance therewith; and, means responsive to saidclock signals for generating said radio frequency assigned to saidunique time slot during said unique time slot.